Stroke is a leading cause of disability in developed countries. At least 25% of persons who suffer stroke will become infected during the course of their acute stroke, and those who become infected experience more disability than those that remain infection free. How systemic infection contributes to neurologic injury during stroke is not well understood. The rationale for the experiments outlined in this proposal is based on the fact that there is a breach in the integrity of the blood-brain barrier (BBB) following stroke that allows the immune system to encounter novel central nervous system (CNS) antigens in both the brain and in peripheral lymphoid organs. The type of immune response generated upon antigen encounter is determined by the composition of the microenvironment at the site of the encounter. The systemic inflammatory response that accompanies an infection could induce the expression ofcostimulatory molecules and alter the context in which antigens are presented to lymphocytes, thus promoting their sensitization to brain antigens. Preliminary data suggestthat animals do develop an autoimmune response to brain following stroke and that lymphocytes sensitized to CNS antigens contribute to cerebral injury, which might explain why infection in the post-stroke period is associated with worse outcome. Manipulating the post-ischemic immune response could therefore be an effective therapeutic intervention for the treatment of stroke. Using an animal model of stroke and a number of in vivo and ex vivo immunologic assays,we plan to confirm and extend our prior findings which show that sensitization to CNS is associated with worse outcome after stroke. The proposed studies will also incorporate sensitive measures of neurological and behavioral performance. More importantly, we hope to show that induction of immunologic tolerance to CNS antigens, even after stroke onset, will prevent the development of CNS autoimmunity; thistolerance should translate to improved outcome from stroke. Given that over 700,000 strokes occur each year in the United States and that at least 175,000 of patients with stroke will develop a concomitant infection, an immune modulating therapy could have significant impact on public health.